Transducer system



NW. 12, 1946. MQS|NNETT 2,411,008

TRANSDUCER SYSTEM F il ed Nov; 28, 1944 CAPACZMNCE AMPLIHER 05' C /LLA TOE 7D REZWFIER I ii/570 CAPACITANCE M AMPLIFIER j OSC/L LA 70/? REC7/F/ER came/72m:-

CHESTER M. SINNETT v A TTOkA/EV +8 IN VEN TOR.

Patented Nov. 12, 1946 TRANSDUCER SYSTEM Chester Sinnett, Westmont, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application November 28, 1944, Serial No. 565,462

7 Claims. 1

My present invention relates generally to transducer systems of the type employing angle modulation of high frequency oscillations, and more particularly to improved frequency modulated (FM) oscillation systems for deriving from reactance variations into electrical currents representative of such variations.

In my application Serial No. 459,375, filed September 23, 1942, I have disclosed systems for translating physical displacements of the mobile electrode of a capacitative reactance into FM oscillations, a simple form of discriminator-rectifier being employed to derive from the FM oscillations electrical waves representativeof the aforesaid physical displacements. In my said application a small condenser was employed whose mobile electrode was stylus-actuated, and the condenser was connected directly across the oscil- 'lator tank circuit. The capacity variation across the tank circuit is limited by the normally small maximum capacitance value of the pickup condenser.

It is one of the primary objects of my present invention to provide a method of amplifying the pickup capacitance variations prior to utilization of the latter in varying the oscillatortank-ci-rcuit frequency.

Another important object of my invention is to convert a small capacitance variation by purely electronic means into a large similar variation, and to employ the large variation :for angle modulating an oscillator.

Another object of my invention is to provide in combination with a high frequency oscillator, a transducer of the reactance type whose variations are utilized to vary the tuned circuit frequency of a reactance tube whose input electrodes are coupled across the oscillator.

.Still another object of my invention is to provides. variable condenser adapted to .be coupled across the tuned circuit of a reactance tube whose input capacitance is connected to the tank circuit of a high frequency oscillator, and the tuned circuit simultaneously acting as .a discriminator of FM oscillations.

A more specific object of my invention is to provide an FM phonograph system whose condense-r pickup unit has its capacitance variations amplified by causing the 'latterto vary the tuning of the resonant plate circuit of a reactance tube whose input capacitance is connected across the oscillator tank circuit of the system.

The novel features which I believe to be characteristic of my invention are set forthwith particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing: Fig. 1 shows an embodiment of the invention; Fig. 1a, showsddeally a resonance curve of :the

discriminator section of the detector;

Fig. 2 shows a simplified modification wherein the FM detector electrodes are embodied in the capacitance amplifier tube; and

Fig. 3 shows a further simplification wherein a special discriminator circuit is eliminated.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, the

embodiment .of Fig. 1' is shown applied to a con.- denser or capacity pickup device. However, there is no restriction to that form of transducer, since the latter. may be a condenser microphone. Additionally, the reactive input element may be any form of reactance provided with a mobile element for providing reactance variations representative of physical changes of force, space, temperature and the like. Assuming for the .purposes of my present patent application that the input element is the capacity pickup device of a record reproducing system, the pickup generally comprises a fixed electrode -.l and a spaced mobile electrode 2.

The spaced electrodes land .2 provide a condenser whose capacity varies in .accordance with the motion of a stylus secured to the mobile electrode 2. The pickup is schematically represented.

It is not a part of this invention, and reference is made to my applicationSerial No. 414,305, filed October 9, 1941, now Patent No. 2,376,456, dated May 22, 1945, for a preferred pickup device.

Displacement of electrode 2 causes a change in the capacitance of the pickup. Displacement of a stylus laterally, as when scanning record grooves, results in a change of the position of electrode 2 with respect to fixed-electrode l The fixed electrode l is represented as the grounded-or low potential side oft-he condenser, while the mobile electrode 2 functions as the high potential side of the pickup device.

Whereas in the system-of my application Serial No. 459,375 the pickup device was connected directly-across the resonant tan-kcircuit of the high frequency oscillator, in the present system capacitance changes of the pickup device are amplified by means of an electronic device which I have termed a capacitance amplifier. The latter electronic device consists of an electron discharge tube 3 which is shown, by way of specific example, as a triode. The tube 3 includes a cathode 4, a control grid 5 and a plate 5. The plate 6 is connected to the +B terminal of a suitable direct current source through a coil 1, and the coil is shunted by a condenser 8. The resonant circuit 1-8 is tuned to substantially the operating normal oscillator frequency F0. The

low potential side of tuned circuit 1-8 is re- 7 turned to ground for high frequency currents by condenser 9. Numeral I0 denotes the coupling path between the mobile electrode 2 and the high potential side of circuit 1-8. The

cathode 4 may be connected to ground through a biasing resistor H which is bypassed for high frequency currents by condenser I2, and the control grid 5 is returned to ground through a resistor l3 which acts as a direct current return path for the control grid.

.It is to be clearly understood that the coupling path In may be of any desired form. For example, it may be a coaxial cable whose grounded sheath would be connected to the fixed electrode l. Merely by way of example, and in no way restrictive upon the invention, F0 may be given a value in the megacycle range such as 30 megacycles (mc.). The control grid 5 of tube 4 has a predetermined capacity with respect to the grounded cathode 4, and numeral [4 denotes this input capacity, or inherent interelectrode capacitance, of the input electrodes of tube 3. The capacity [4 is designated in dotted lines to represent the fact that it is inherent, and is not an actual physical condenser across the tank circuit of the high frequency oscillator. The control grid 5 is connected to the high potential side of tank circuit I5 of the high frequency oscillator by means of a direct current blocking condenser I6. The low potential side of tank circuit I5 is grounded. Tank circuit l5 consists of coil I! and and shunt condenser I8, and the tank circuit is normally tuned to the frequency F0.

The oscillator tube is designated by numeral l9 and is shown as a pentode type tube, although my invention is not limited to that type of tube. Since the present invention is concerned mainly with the circuit details between the reactive input element l-2 and the oscillator tank circuit l'l-l8, the remainder of the system comprising the oscillator and discriminator-rectifier will be generally referred to.

The oscillator tube cathode is connected to a suitable point on coil [1, control grid 21 being connected by direct current blocking condenser 22 to the high potential side of tank circuit I5. A suitable grid leak resistor 23 returns grid 2| to ground. Plate 24 is connected through coil 25 to the +3 terminal of a suitable direct current source. Coil 25 is shunted by condenser 26, and the resonant circuit 25-26 is tuned to a frequency which is sufiiciently different from the normal oscillator frequency F0 to provide frequency discriminator action. In Fig. lo I have shown an ideal resonance curve of circuit 25-26. The peak frequency of the resonance curve is chosen to be Fo+f, the value of 1 being such that F0 falls at the midpoint of the most linear section of one of the flanks of the resonance curve. If, for example, F0 is chosen to be me, then .the discriminator circuit 25-26 will be tuned to a peak frequency sufficiently above 30 me. so that the latter value will fall at a linear point of the lower flank of the resonance curve shown in Fig. 1a. Of

4 course F0 may be located on a suitable point of the opposite flank of the resonance curve.

Those skilled in the art of FM communication are fully aware of the fact that the oscillations produced in the tank circuit I5 are of substantially constant frequency in the absence of capacitance variation of the pickup I-2. However, variation of the pickup capacitance causes the amplified capacitance I4 to vary the frequency of tank circuit l5, and, of course, the frequency variation or deviation will correspond to the capacitance variations at the pickup I-2. Variations in frequency of the local oscillations will result in developing across circuit 25-26 high frequency oscillations whose amplitude will vary in accordance with the frequency variations occurring at tank circuit l5. This is the well known frequency discrimination action which has been described in my aforesaid co-pending patent application.

Any suitable type of rectifier may be coupled across circuit 25-26, as by coupling condenser 21, for the purpose of rectifying the high frequency oscillations of variable amplitude. The rectified voltage produced during the rectification step corresponds to the capacitance variations of the pickup element l-2. The rectifier device coupled to the discriminator circuit 25-26 may be of any well known construction, as, for example, a simple diode. Where the mobile electrode 2 is varied by a stylus scanning sound grooves of a record, the voltage produced by the rectifier coupled to circuit 25-26 represents the audio frequency voltage corresponding to the recorded sound. In the case where'condenser l-2 is a condenser microphone the output of the system Will be the audio frequency waves corresponding to the sound waves varying the capacitance of condenser l-2.

In order to provide an amplification of the relatively small capacity changes of condenser 1-2 the tube 3 and its associated circuits are utilized. Since the resonant circuit 1-8 of tube 3 is tuned to F0, it follows that tube 3 will have a plate circuit whose plate impedance is essentially resistive when circuit 1-8 is tuned to F0. It can be shown that in any tube, such as tube 3 of Fig. 1, whose plate impedance consists of a resonant circuit tuned to the frequency of the high frequency voltage applied to the grid of the tube, the input capacitance M will have its m gnitude depend upon the effective magnitude of the plate impedance of tube 3. At resonance the input capacity C1 of a vacuum tube amplifier is given by:

where M is the amplification factor of the tube; Cgk is the capacity between the grid and cathode; and Cgp is the' capacity between cathode and plate. This relation applies for a resistance load. Since at resonance a tuned circuit possesses unity power factor, the relation will hold for the circuit shown in Fig, l for tube 3. If now the frequency of the resonant plate circuit of tube 3 is varied relative to F0 the effective impedance of the plate circuit varies. On either side of resonance (F0) the value of Cl decreases, and for the specified circuit is given by:

Due to this effect the input capacity IA of tube 3 is reflected into the oscillator tank circuit in magnified form relative to the capacitance of condenser l, 2.

2, 91 mos This change in effect" of plate impedance of tube 3 will be clearly magnified as a correspond ing variation in the, magnitude of the input capacitance 14, and the variation of capacitance I4 will follow the frequency variation of circuit 'l'--8. It is for this reason that the capacity variation of capacitance M will frequency modulate the" high frequency oscillations produced at tank circuit [5; The extent of frequencydeviaam at tank circuit l5 will'be a function of the amplitude of physical displacement of mobile electrode 2, while the rate of frequency variation will depend upon the rate of physical displacement. In the case of recorded sound the extent of frequency variation at tank circuit IE will depend upon the amplitude of the recorded sound, aridthe rate of deviation will depend upon the audio' frequencies per se] The generic term anglemodulated is'employed herein to indicate the fact that the variations of the-high frequency-energy' at tank circuitifi could be phases modulated, since frequency and phase modulatiorr have characteristicsin common.

In Fig. 2 there is shown a modification of the invention wherein the rectifier device coupled across discriminator circuit 25-2i5 has its electrodes included within the tube envelope of the capacitance amplifier. Additionally, the leads it are an unshielded pair of twisted wires. It will further be noticed that control grid 5 of tube 3' is connected directly to the high potential end of oscillator tank'coil i1. Adjacent a portion of the cathode 4 of tube 3 there is located the diode anode 3i! which is returned to the grounded cathode through a path consisting of the choke coil 3! and load resistor 32, the latter resistor being bypassed for; high frequency currents by condenser 33. The anode 38 is connected through direct current blocking condenser 21 to the plate 'end of discriminator coil 25. The tube 3 may be a diode-triode of the 6Q7G type, although any other well known type of multiple duty tube may be employed. The rectified audio frequency voltage across resistor 32 may be transmitted by coupling, condenser 3 toany suitable audio frequency amplifier network. The action described in connection with Fig. 1 applies equally well to the functions of the system of Fig. 2. Here again the inherent input capacity of tube 3 is Varied in accordance with the variations in magnitude of condenser [-2, and the input capacitance of the triode section of tube 3' is provided across the oscillator tank circuit as described for the system of Fig. 1.

In Fig. 3 I have shown a further simplification of the invention. In this case discriminator circuit 2526 is completely dispensed with, while the resonant plate circuit of tube 3' is caused to perform frequency discrimination in addition to its function as described in Fig. 1. modified system the resonant plate circuit 1, 8 of tube 3' is tuned to the peak frequency Fo+f, while the oscillator tank circuit ll, l8 of tube i9 is tuned to F0. The variations in capacitance of condenser i, 2 cause corresponding variations in plate impedance of tube 3' thereby producing amplified capacitance variations across tank circuit IT, IS. The FM oscillations across tank circuit I1, is are applied to grid 5, and there will be developed in the plate circuit of tube 3' FM oscillations which are applied to circuit 1, 8. Since circuit 7, 8 is tuned to Fo-land the FM oscilla- In this tions have a mean or center frequency F0, it

follows that circuit 1, 8 acts in the manner of a lations into amplitude V modulated oscillations. fll ie latter are appliedtoanode 30 of diode recti fieriril, A throi-igh condenser 45 which has a small c The anode 30 is connected to ground through a series path consisting of 'choke coil 3| and load resistor 32. Load resistor 32 is shuntedby condenser. 33 which bypasses high frequency currents. The rectified voltage across-resistor 32 is applied through condenser 34 to the following audio frequency network. The fact that: circuit 7, 8 is tuned to Fo-l-f in no way interferes with the function of the circuit in acting as the plate impedance 'of tube 3''. Changes in pickup capacity modulate the high frequency oscillations, but the amplified oscillator voltage also swings up anddown the flank'ofthe resonance curve of tuned circuitT, 8. This causes a change in voltage across thediode load resistor 32.

Tube 3 is fundamentally a high frequency amplifier receiving its energy from oscillator tube l9. Therewill appear across the tank circuit: 1, 8"aniplified energy derived from the oscillator. The input capacity of tube 3' is,however, connectedacross tank circuit l1, l3. From the above relation for C'i'the tuning o-f'the plate circuit of tube 3 has an effect on its input capacity. Even when circuit 1, 8 is tuned ofi resonance with Fe it can be: seen fromtheabove formula that the eifectiveinput capacity becomes a variabie'or a magnitude larger than the tubes normal grid to cathode capacity. The variation in frequency of the oscillator isamplified by tube 3', and-with its plate-circuitoff resonance large changes in voltage occur across circuit 1 8. The capacity. variations in circuit l, 8 dueto the pick- -up aresmall, but the changes in voltageare large since tube 3- amplifies the oscillator output "voltage at Foif. l

systems for carrying my invention-into effect, it

organizations shown and described,but that many modifications-may be made: without-.departing from the scop'e of my invention.

What I claim is: v r

'1. In combination, an oscillator having a resonant tank circuit tuned to a desired frequency, a reactance device having a displaceable element responsive to a physical force for varying the reactance of said device, and the improvement which comprises an electron discharge tube having at least a cathode, control grid and plate, a

, second resonant circuit connected beween the cathode andplate tuned substantially to. said desired frequency, means electrically connecting the grid to cathode capacity of said tube across said tank circuit, means connecting the reactance device across said second resonant circuit for providing frequency variations of the latter in response to displacements of said element thereby to cause corresponding variations of said grid to cathode capacity, and means for deriving from oscillations variable in frequency, due to said capacity variations, a Voltage representative of said physical force.

2. In combination with a high frequency oscillator producing oscillations of a predetermined frequency, a condenser having a mobile electrode responsive to forces to be reproduced, a tube having a resonant plate circuit tuned substantially to said frequency, said condenser being connected directly across said resonant plate circuit to vary the frequency thereof in accordance with move- Whi'le I. have indicated and described several mentsof the mobile, electrode, means responsive to corresponding variations in'the input capacitanceof said tube for varying the frequency of said oscillations, and means for deriving from the frequency-variable oscillations a voltage whose variations are, representative of said electrode movements.

3. In combination with a high frequency oscillator producing oscillations of a predetermined frequency, a condenser having a mobile electrode responsive to forces to be reproduced, a tube having a resonant plate circuit tuned substantially to said frequency, said condenser being connected directly across said resonant plate circult to vary the frequency thereof in accordance with movements of the mobile electrode, means responsive to corresponding variations in the input capacitance of said tube for varying the frequency of said oscillations, means for deriving from the frequency-variable oscillations a voltage whose variations are representative of said electrode movements, said mobile electrode having a record reproducing stylus coupled thereto, and said deriving means including a rectifier whose input circuit is said resonant plate circuit,

4. In combination with a high frequency oscillator producing oscillations of a predetermined frequency, a condenser having a mobile electrode responsive to forces to be reproduced, a tube having a resonant plate circuit tuned substantially to said frequency, said condenser being connected directly across said resonant plate circuit to vary the frequency thereof in accordance with movements of the mobile electrode, mean responsive to corresponding variations in the input capacitance of said tube for varying the frequency of said oscillations, means for deriving from the frequency-variable oscillations a voltage whose variations are representative of said electrode movements, said deriving means comprising a rectifier whose electrodes are included in the tube envelope of said tube, and a frequency discriminator circuit coupling said oscillator and rectifier and tuned to a frequency close to said predetermined frequency. l

5. In combination, an oscillator tube having a resonant tank circuit tuned to a desired high frequency, a reactance device having a displaceable element responsive to a physical force for varying the reactance of said device, an electron discharge'tube having at least a cathode, control grid and plate, a second resonant circuit connected between the cathode and plate tuned to a frequency differing by a small value from said desired frequency, means electrically connecting the grid to cathode capacity of said tube across said tank circuit, means connecting the reactance device across said second resonant circuit for providing frequency variations of the latter in response to displacements of said element thereby to cause corresponding variations of said grid to cathode capacity, and rectifier means in circuit with said second resonant circuit for deriving from oscillations variable in frequency, due to said capacity variations, a voltage representative .ofsaid physical force,

6. In combination with a high frequency oscillator tube producing oscillations of a predetermined frequency, a condenser pickup having a mobile electrode responsive to record grooves to be reproduced, a tube having a resonant plate circuit tuned oif resonance with said frequency, said pickup condenser being connected directly across said resonant plate circuit to vary the frequency thereof in accordance with movements of the mobile electrode, means responsive to corresponding variations in the input capacitance of said tube for varying the frequency of said oscillations, a rectifier whose electrodes are included in the tube envelope of said tube, and a condenser coupling said resonant plate circuit to said rectifier.

7. In combination with a high frequency oscillator, a tube whose input capacitance is coupled across a frequency determining circuit of the oscillator, a resonant circuit tuned away from the oscillator frequency by a small frequency value connected in the tube plate circuit, and a modulating reactance device connected directly across said resonant circuit,

CHESTER M. SINNETT. 

